fix: preserve function length when lowering params

packages/babel-plugin-proposal-destructuring-private/src/index.ts

@@ -15,6 +15,7 @@ import { types as t } from "@babel/core";
 
 const {
   assignmentExpression,
+  assignmentPattern,
   cloneNode,
   expressionStatement,
   isExpressionStatement,
@@ -42,19 +43,35 @@ export default declare(function ({
       // (b, { #x: x } = I) => body
       // transforms to:
       // (b, p1) => { var { #x: x } = p1 === undefined ? I : p1; body; }
-      const index = path.node.params.findIndex(param => hasPrivateKeys(param));
-      if (index === -1) return;
+      const firstPrivateIndex = path.node.params.findIndex(param =>
+        hasPrivateKeys(param),
+      );
+      if (firstPrivateIndex === -1) return;
       // wrap function body within IIFE if any param is shadowed
       convertFunctionParams(path, ignoreFunctionLength, () => false, false);
       // invariant: path.body is always a BlockStatement after `convertFunctionParams`
       const { node, scope } = path;
       const params = node.params;
-      const paramsAfterIndex = params.splice(index);
+      const firstAssignmentPatternIndex = ignoreFunctionLength
+        ? -1
+        : params.findIndex(param => param.type === "AssignmentPattern");
+      const paramsAfterIndex = params.splice(firstPrivateIndex);
       const { params: transformedParams, variableDeclaration } =
         buildVariableDeclarationFromParams(paramsAfterIndex, scope);
 
       path.get("body").unshiftContainer("body", variableDeclaration);
       params.push(...transformedParams);
+      // preserve function.length
+      // (b, p1) => {}
+      // transforms to
+      // (b, p1 = void 0) => {}
+      if (firstAssignmentPatternIndex >= firstPrivateIndex) {
+        params[firstAssignmentPatternIndex] = assignmentPattern(
+          // @ts-ignore The transformed assignment pattern must not be a RestElement
+          params[firstAssignmentPatternIndex],
+          scope.buildUndefinedNode(),
+        );
+      }
       scope.crawl();
       // the pattern will be handled by VariableDeclaration visitor.
     },
@@ -84,6 +101,11 @@ export default declare(function ({
         // for (const { #x: x } of cls) body;
         // transforms to:
         // for (const ref of cls) { const { #x: x } = ref; body; }
+        // todo: the transform here assumes that any expression within
+        // the destructuring pattern (`{ #x: x }`), when evluated, do not interfere
+        // with the iterator of cls. Otherwise we have to pause the iterator and
+        // interleave the expressions.
+        // See also https://gist.github.com/nicolo-ribaudo/f8ac7916f89450f2ead77d99855b2098
         const temp = scope.generateUidIdentifier("ref");
         node.left = variableDeclaration(left.kind, [
           variableDeclarator(temp, null),

packages/babel-plugin-proposal-destructuring-private/test/fixtures/assumption-ignoreFunctionLength/function-params/input.js

@@ -0,0 +1,7 @@
+class C {
+  static #x;
+  static 0(...[...{0: { #x: x }}]) {}
+  static 1(a, b = 1, { #x: x }, ...c) {}
+  static 2(a, b, { #x: x } = C) {}
+  static 3(a, b, { #x: x }, c = 1) {}
+}

packages/babel-plugin-proposal-destructuring-private/test/fixtures/assumption-ignoreFunctionLength/function-params/output.js

@@ -0,0 +1,26 @@
+class C {
+  static 0(..._p) {
+    var [..._p2] = _p,
+        x = babelHelpers.classStaticPrivateFieldSpecGet(_p2[0], C, _x);
+  }
+
+  static 1(a, b = 1, _p3, ..._p4) {
+    var x = babelHelpers.classStaticPrivateFieldSpecGet(_p3, C, _x),
+        c = _p4;
+  }
+
+  static 2(a, b, _p5) {
+    var x = babelHelpers.classStaticPrivateFieldSpecGet(_p5 === void 0 ? C : _p5, C, _x);
+  }
+
+  static 3(a, b, _p6, _p7) {
+    var x = babelHelpers.classStaticPrivateFieldSpecGet(_p6, C, _x),
+        c = _p7 === void 0 ? 1 : _p7;
+  }
+
+}
+
+var _x = {
+  writable: true,
+  value: void 0
+};

packages/babel-plugin-proposal-destructuring-private/test/fixtures/assumption-ignoreFunctionLength/options.json

@@ -0,0 +1,12 @@
+{
+  "assumptions": {
+    "ignoreFunctionLength": true
+  },
+  "plugins": [
+    "proposal-destructuring-private",
+    "proposal-class-static-block",
+    "proposal-class-properties",
+    "proposal-private-methods",
+    ["proposal-object-rest-spread", { "useBuiltIns": true }]
+  ]
+}

packages/babel-plugin-proposal-destructuring-private/test/fixtures/function-params--es2015/function-length/exec.js

@@ -1,5 +1,11 @@
 class C {
-  #x;
-  static m(a, { #x: x }, ...b) {}
+  static #x;
+  static 0(...[...{0: { #x: x }}]) {}
+  static 1(a, b = 1, { #x: x }, ...c) {}
+  static 2(a, b, { #x: x } = C) {}
+  static 3(a, b, { #x: x }, c = 1) {}
 }
-expect(C.m.length).toBe(2)
+expect(C[0].length).toBe(0);
+expect(C[1].length).toBe(1);
+expect(C[2].length).toBe(2);
+expect(C[3].length).toBe(3);

packages/babel-plugin-proposal-destructuring-private/test/fixtures/function-params--es2015/function-length/input.js

@@ -0,0 +1,7 @@
+class C {
+  static #x;
+  static 0(...[...{0: { #x: x }}]) {}
+  static 1(a, b = 1, { #x: x }, ...c) {}
+  static 2(a, b, { #x: x } = C) {}
+  static 3(a, b, { #x: x }, c = 1) {}
+}

packages/babel-plugin-proposal-destructuring-private/test/fixtures/function-params--es2015/function-length/output.js

@@ -0,0 +1,26 @@
+class C {
+  static 0(..._p) {
+    var [..._p2] = _p,
+        x = babelHelpers.classStaticPrivateFieldSpecGet(_p2[0], C, _x);
+  }
+
+  static 1(a, b = 1, _p3, ..._p4) {
+    var x = babelHelpers.classStaticPrivateFieldSpecGet(_p3, C, _x),
+        c = _p4;
+  }
+
+  static 2(a, b, _p5 = void 0) {
+    var x = babelHelpers.classStaticPrivateFieldSpecGet(_p5 === void 0 ? C : _p5, C, _x);
+  }
+
+  static 3(a, b, _p6, _p7 = void 0) {
+    var x = babelHelpers.classStaticPrivateFieldSpecGet(_p6, C, _x),
+        c = _p7 === void 0 ? 1 : _p7;
+  }
+
+}
+
+var _x = {
+  writable: true,
+  value: void 0
+};

packages/babel-plugin-proposal-destructuring-private/test/fixtures/function-params/function-length/exec.js

@@ -1,5 +1,11 @@
 class C {
-  #x;
-  static m(a, { #x: x }, ...b) {}
+  static #x;
+  static 0(...[...{0: { #x: x }}]) {}
+  static 1(a, b = 1, { #x: x }, ...c) {}
+  static 2(a, b, { #x: x } = C) {}
+  static 3(a, b, { #x: x }, c = 1) {}
 }
-expect(C.m.length).toBe(2)
+expect(C[0].length).toBe(0);
+expect(C[1].length).toBe(1);
+expect(C[2].length).toBe(2);
+expect(C[3].length).toBe(3);

packages/babel-plugin-proposal-destructuring-private/test/fixtures/function-params/function-length/input.js

@@ -0,0 +1,7 @@
+class C {
+  static #x;
+  static 0(...[...{0: { #x: x }}]) {}
+  static 1(a, b = 1, { #x: x }, ...c) {}
+  static 2(a, b, { #x: x } = C) {}
+  static 3(a, b, { #x: x }, c = 1) {}
+}

packages/babel-plugin-proposal-destructuring-private/test/fixtures/function-params/function-length/output.js

@@ -0,0 +1,23 @@
+class C {
+  static #x;
+
+  static 0(..._p) {
+    var [..._p2] = _p,
+        x = _p2[0].#x;
+  }
+
+  static 1(a, b = 1, _p3, ..._p4) {
+    var x = _p3.#x,
+        c = _p4;
+  }
+
+  static 2(a, b, _p5 = void 0) {
+    var x = (_p5 === void 0 ? C : _p5).#x;
+  }
+
+  static 3(a, b, _p6, _p7 = void 0) {
+    var x = _p6.#x,
+        c = _p7 === void 0 ? 1 : _p7;
+  }
+
+}